1. Field of the Invention
The present invention relates to photoresists having high thermal stability in terms of plastic flow. The photoresists are comprised of structures having recurrent acid labile or photolabile groups, such that in the deprotected state (with the acid labile or photolabile group removed) they have a hydrogen bond donor and a free hydrogen bond acceptor. Thus, a hydrogen-bonded network is generated upon deprotection which requires energy to break. The recurrent acid labile or photolabile groups are typically pendant to the polymeric backbone.
2. Background Art
Processing of semiconductor devices frequently requires the use of temperatures as high as 200.degree. C.; as a result, it is important to have resist materials which can withstand such temperatures without the occurrence of plastic flow which distorts the dimensions of the resist structure.
In U.S. Pat. No. 3,779,778, Smith et al. disclosed novel photosolubilizable compositions comprising (1) a water-insoluble compound containing one or more acid-degradable groups, and (2) a photoinitiator comprising a photolyzable acid progenitor.
In U.S. Pat. No. 4,491,628, resists sensitive to UV, electron beam and X-ray radiation with positive or negative tone upon proper choice of a developer are formulated from a polymer having recurrent pendant groups such as tert-butyl ester or tert-butyl carbonates that undergo efficient acidolysis with concomitant changes in polarity (solubility) together with a photoinitiator which generates acid upon radiolysis. A sensitizer component that alters wavelength sensitivity may also be added. The preferred acid labile pendant groups are tert-butyl esters of carboxylic acids and tert-butyl carbonates of phenols but, it is understood that a wide range of acid labile groups are operative in the invention. These include trityl, benzyl, benzhydryl modifications as well as others well known in the art.
Developed resist structures generated from the above, previously disclosed, preferred t-butyl carbonates of phenols such as poly(p-tert-butoxycarbonyloxy-methylstyrene), poly(p-tert-butoxycarbonyloxystyrene), or poly(tert-butyl p-isoprophenylphenyloxyacetate) exhibit a thermal stability (in terms of plastic flow) of less than about 160.degree. C. as measured by T.sub.g (determined by TGA at a heating rate of about 10.degree. C./min).
The previously disclosed, preferred t-butyl ester of a carboxylic acid, poly(t-butyl p-vinylbenzoate) demonstrates a high opacity below about 280 nm, and thus is not useful for a single layer processes utilizing imaging via deep UV radiation; deep UV imaging being preferred for numerous reasons known in the art of lithography.
Developed resist structures generated from the previously disclosed, preferred t-butyl esters of carboxylic acids such as poly(tert-butyl methacrylate) form anhydride linkages which can crosslink upon heating so that the resist becomes insoluble in the common lithographic developers and strippers. Thus, it may not be possible to develop the latent image in the resist layer, or, if subsequent to developing, processing temperatures rise above about 160.degree. C., the developed resist structure may not be removable from the underlaying substrate.
Thus, it would be desirable to have a resist material which is sufficiently transparent to deep UV (wavelengths below about 280 nm) prior to imaging, to permit deep UV imaging; which is thermally stable after imaging to processing temperatures greater than 160.degree. C.; and, which does not crosslink upon heating to these same processing temperatures, to become insoluble in the common lithographic developers or strippers.